<!DOCTYPE html>
<!--
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<html>
    <head>
        <meta charset="UTF-8">
        <title></title>
    </head>
    <body>
        <?php
        // isset(变量) 用来检测变量是否存在，存在返回true ，不存在返回false
        $a  = array ( 'test'  =>  1 ,  'hello'  =>  NULL ,  'pie'  => array( 'a'  =>  'apple' ));

        var_dump (isset( $a [ 'test' ]));             // TRUE
        var_dump (isset( $a [ 'foo' ]));              // FALSE
        var_dump (isset( $a [ 'hello' ]));            // FALSE
        ?>
    </body>
</html>
